The synthesis and localization of the components of vertebrate photoreceptor cell (rod) outer segments, particularly the principal protein, opsin, is studied by "pulse" addition of radioactive precursors (amino acids, sugars, lipids) followed by "chase" addition of nonradioactive precursors to intact bovine foveas (retinas) incubating in vitro in a complex liquid tissue culture medium. At intervals after the "pulse," subcellular fractions of the foveas are obtained by isopycnic centrifugation of tissue homogenates in linear sucrose density gradients. The fractions are enzymatically defined, and precursors to the outer segments will be identified by polyacrylamide gel electrophoresis, isolation of characteristic peptides and gas chromatography. Our aim is to determine where opsin is synthesized (free or membrane bound ribosomes), where the phospholipids are added (particularly, 22:6 polyunsaturated fatty acids), and how these insoluble components are transported and assembled into the rod outer segment disc membranes.